The present invention relates to a ceramic seal assembly for sealing and electrically insulating a terminal extending through the housing of an electrical device, and more particularly, to an adapter for hermetically securing a ceramic seal assembly to a housing, for example a battery housing such as a hydroformed pressure vessel used to house the components of a metal gas cell.
Relatively light weight, thin walled pressure vessels have been employed to house electrical devices, especially the components of a metal gas cell. For example, nickel hydrogen batteries include a plate stack encased within a sealed metal vessel. Conventionally, a sheet of metal, such as a nickel alloy, is hydroformed into a hollow cylindrically shaped member having one hemispherically configured end portion by stretching the sheet around an appropriately shaped mandrel. Two such members are welded together to form a cylindrically shaped vessel or casing having two hemispherically configured end portions. As thus conventionally manufactured, the pressure vessel walls have a thickness of about 0.020-0.050 inch. A terminal is electrically connected to the leads carrying a like electrical charge which emanate from one end of the plate stack and extends through an aperture provided in one of the end portions. Metal gas cells are typically charged with a gas under a relatively high pressure, such as hydrogen at about 700-1200 psig.
Conventionally, the hydroformed end portion of a metal gas pressure vessel has a formed neck portion which defines an aperture through which a terminal extends. A seal against fluid leakage between the vessel and the terminal has been accomplished in one of two manners. First, a substantially annular TEFLON plug is positioned around the terminal and is forced into the neck of the vessel. Such force causes the TEFLON plug to compress and flow to create a seal between the neck of the vessel and the terminal. Secondly, a substantially annular TEFLON plug is positioned around the terminal and is sized to be positioned within the neck of the vessel without being compressed. Thereafter, the neck is crushed, such as by crimping, to deform the TEFLON thereby causing the TEFLON to flow and to create a seal.
One problem associated with the use of a TEFLON seal is that the coefficient of thermal expansion of TEFLON is an order of magnitude greater than that of the metal of which the terminal or the pressure vessel is constructed. Pressure and thermal cycling during the life of a metal gas cell may cause a particular TEFLON seal to flow from 0.007-0.010 inch. Such flow creates the potential for development of a fluid leak, and therefore premature failure of an electrical device housed within the vessel. Another problem is that temperatures of 300.degree. C. or greater which may be applied externally to terminals so as to solder external leads thereto will cause TEFLON seals to melt.
Accordingly, it is an object of the present invention to provide for a seal assembly for an electrical device housed within a thin walled pressure vessel which will seal and electrically insulate a terminal extending through the vessel against fluid leakage over repeated pressure and thermal cycling.
Another object of the present invention is to provide for a seal assembly for an electrical device housed within a thin walled pressure vessel which can withstand the temperature associated with soldering external leads to a terminal extending through the vessel without any alteration in structure or function.
A further object of the present invention is to provide an adapter for hermetically securing a seal assembly to a pressure vessel housing an electrical device which is capable of withstanding repeated high pressure and thermal cycling without failing.